Frank -
OK. The RF Output meter is not sensitive enough to balance the Balanced
Modulator.
Tune up in the TUNE position on LSB, then switch to CW and close PTT.
Then listen on an external receiver at the transmitter frequency for the
"suppressed" carrier. Set AVC to FAST, and adjust the Carrier Balance
control (10 turn pot) and C6 for minimum S-Meter reading.
What we are doing is adjusting the Balanced Modulator to "balance" the
division of the LO signal and compensating for the stray capacitances in
that circuit. The BM's job is to "mix" the CO and the modulation,
resulting in a signal composed of the SUM and DIFFERENCE (just like a
mixer). What we then have is an AM signal with a carrier and two AF
sidebands. The "balance" part comes in to remove the carrier of this AM
signal by nulling it out. This leaves us with a DSB or Double Side Band
(suppressed carrier) signal, or the two AF sidebands WITHOUT a carrier.
So the object is to get the carrier as low as possible. For SSB, the CO
sits in the valley between the two filters, improving the Carrier
Suppression even more.
CW operation is accomplished by shifting the CO so that it is no longer
in the valley, but fully within the LSB filter passband. Then a DC
current is applied to half the diodes in the BM to "unbalance" it and
allow the CO to pass through, on through the filter and the rest of the
way to the antenna.
This is the reason we key the PTT instead of just closing the KEY, we
want to turn the CO on, put the transmitter in transmit, _without_
applying the DC current to the CO to "unbalance" it. This is the normal
condition for SSB, where the carrier is suppressed as much as possible,
less the loss of the filter, so we can "hear" it and adjust the balance
to minimize it.
73, Garey - K4OAH
Glen Allen, VA
Drake 2-B, 4-B, C-Line& TR-4/C Service Supplement CDs
<www.k4oah.com>
iz oos wrote:
Hi!
I did my homework! Indeed, the carrier oscillator crystal frequency
was not exactly set to 5.645 MHz, mainly due to C2 which was offest by
30 degree and a bit also to T14 which was not completely peaked.
Switching sideband switch back and forth it was easy to note
difference in plate current and Adjust C-2 to make the current and
output of the sidebands equal. Current was about 0.8 and output zero
or close to zero.
Plugging the mike, both sidebands now sound much more the same,
however I had still to rebalance the carrier balance control which was
set fully counter clockwise. Setting the function switch to CW and and
PTT on mic ON the relative output meter stays at zero for whatever
adjustment of the carrier balance control !!!! So, I CAN'T rebalance
the Carrier Balance control !!!!
Why is that???
73,
Frank IZ2OOS
2010/3/21 Garey Barrell<k4...@mindspring.com>:
Frank -
Start with the Carrier Oscillator, Filter Match& Balanced Modulator
alignment in Section VII, Section B. The idea is to get the Carrier
Oscillator "centered" in the valley between the two filters as shown at "0"
on Page 37.
73, Garey - K4OAH
Glen Allen, VA
Drake 2-B, 4-B, C-Line& TR-4/C Service Supplement CDs
<www.k4oah.com>
iz oos wrote:
Hi Folks,
I am testing my T-4XB. Before going in the air, I wanted to check my
modulation first. Using different high impedance microphones I
transmitted into a dummy load and listened to the receiver without
antenna. I found that the LSB has a different modulation compared to
the USB. The USB is warmer and stronger. The LSB seems to me with many
highs and without basses. The USB is very clear and according to my
average output meter has more average power than the LSB mode.
Listening with the wrong sideband, in LSB when the transmitter was in
USB, I could still copy my modulation. Instead, listening to USB with
the transmitter is LSB, I could hear nothing at all.
Do you have any idea where I could have a look?
73,
Frank IZ2OOS
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